Known in the prior art are numerous angiogenic growth factors such as factors HARP, MK, FGF-1, FGF-2, VEGF, HIV1-tat, HIV2-tat, HGF, HB-EGF and angiogenin. Among these factors, HARP (Heparin Affin Regulatory Peptide), which is also called PTN (pleiotrophin) or HB-GAM (heparin binding-growth associated molecule), constitutes with MK (Midkine) a family of structurally related growth/differentiation factors that bind to heparin and having 50% homology in amino acids [1, 2].
The growth factor HARP is a polypeptide of 168 amino acids containing an N-terminal hydrophobic motif of 32 amino acids corresponding to a signal peptide. In its mature form, HARP is a secreted protein of 136 amino acids in its short form or 139 amino acids in its long form, whose apparent molecular weight, determined in SDS-PAGE under reducing conditions, is 18 kDa.
HARP was initially isolated from rat neonate brains as a molecule inducing in vitro neurite growth [3], suggesting that this polypeptide is involved in the maturation of neuronal cells [4]. Subsequent studies showed that this polypeptide was also present in non-neuronal tissues, such as the heart [5], uterus [6], cartilage [7] and bone extracts [8], demonstrating that the function of HARP is not limited to a promotional action on neurite growth as previously reported [3].
HARP is capable of stimulating the growth of fibroblastic, epithelial and endothelial cells in vitro [6, 9]. This mitogenic action has since been confirmed by the use of recombinant proteins produced from eukaryote expression systems [9, 12]. HARP also induces in vitro the formation of pseudocapillaries. In vivo, in different tissue models, localization of HARP is especially associated with endothelial cells of blood capillaries [16]. The data concerning HARP available at present suggest that this polypeptide plays a role in the complex mechanisms involved in angiogenesis and in tumor neoangiogenesis. Extensive research has been performed with regard to this aspect to determine the involvement of HARP in tumoral progression, particularly, in hormone-dependent tumors such as the breast and prostate.
Studies pertaining to the biological properties of HARP have been performed by numerous laboratories [2] and, despite much debated results, it appears that HARP, like MK, is involved in the control of cellular proliferation [2, 9-11]. Moreover, it has been demonstrated that human purified recombinant HARP proteins are mitogenic for endothelial cells [9, 12] and exert in vitro an angiogenic action [12]. Numerous studies have shown the involvement of HARP and MK in developmental processes [10, 13, 14]. Studies of the distribution of HARP protein mRNA during embryonic and postnatal development suggest important functions in cell growth and differentiation [15]. Nevertheless, the physiological functions in vivo of these molecules is still poorly understood. The presence of HARP transcripts in adult tissues including the meninges, iris, testicles and uterus also indicates a physiological role in adult age.